Abstract: We consider both the defocusing and focusing cubic nonlinear Klein-Gordon equations

in two space dimensions for real-valued initial data and . We show that in the defocusing case, solutions are global and have finite global spacetime bounds. In the focusing case, we characterize the dichotomy between this behaviour and blowup for initial data with energy less than that of the ground state.

These results rely on analogous statements for the two-dimensional cubic nonlinear Schrödinger equation, which are known in the defocusing case and for spherically-symmetric initial data in the focusing case. Thus, our results are mostly unconditional.

It was previously shown by Nakanishi that spacetime bounds for Klein-Gordon equations imply the same for nonlinear Schrödinger equations.